The present invention relates to compositions and methods for drilling well bores in subterranean formations. More particularly, in certain embodiments, the present invention relates to drilling fluids with a weighting agent that comprises sub-micron precipitated barite.
Natural resources such as oil or gas residing in a subterranean formation can be recovered by drilling a well bore that penetrates the formation. During the drilling of the well bore, a drilling fluid may be used to, among other things, cool the drill bit, lubricate the rotating drill string to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to the entrance into the well bore of formation fluids, and remove drill cuttings from the well bore. A drilling fluid may be circulated downwardly through a drill pipe and drill bit and then upwardly through the well bore to the surface.
In order to prevent formation fluids from entering the well bore, the hydrostatic pressure of the drilling fluid column in the well bore should be greater than the pressure of the formation fluids. The hydrostatic pressure of the drilling fluid column is a function of the density of the drilling fluid and depth of the well bore. Accordingly, density is an important property of the drilling fluid for preventing the undesirable flow of formation fluids into the well bore. To provide increased density, weighting agents are commonly included in drilling fluids. Weighting agents are typically high-specific gravity, finely ground solid materials. As referred to herein, the term “high-specific gravity” refers to a material having a specific gravity of greater than about 2.6. Examples of suitable weighting agents include, but are not limited to, barite, hematite, ilmentite, manganese tetraoxide, galena, and calcium carbonate.
As well bores are being drilled deeper, the pressure of the formation fluids increases. To counteract this pressure increase and prevent the undesired inflow of formation fluids, a higher concentration of weighting agent may be included in the drilling fluid. However, increasing the concentration of weighting agent may be problematic. For example, as the concentration of the weighting agent increases problems with particle sedimentation may occur (often referred to as “sag”). Among other things, particle sedimentation may result in stuck pipe or a plugged annulus. Particle sedimentation may be particularly problematic in directional drilling techniques, such as horizontal drilling. In addition to particle sedimentation, increasing the concentration of the weighting agent also may undesirably increase the viscosity of the drilling fluid, for instance. While viscosification of the drilling fluid may be desired to suspend drill cuttings and weighting agents therein, excessive viscosity may have adverse effects on equivalent circulating density. For example, an undesirable increase in the equivalent circulating density may result in an undesired increase in pumping requirements for circulation of the drilling fluid in the well bore.
Several techniques have been utilized to prevent undesired particle sedimentation while providing a drilling fluid with desirable rheological properties. For instance, decreasing the particle size of the weighting agent should create finer particles, reducing the tendency of the particles to settle. However, the inclusion of too many particles of a reduced particle size typically causes an undesirable increase in viscosity. Accordingly, the use of particle sizes below 10 microns has typically been avoided. This is evidenced by the API specification for barite as a drilling fluid additive, which limits the % w/w of particles below 6 microns to a 30% w/w maximum to minimize viscosity increase.
One approach to reducing particle size while maintaining desirable rheology involves utilizing particles of a reduced size while avoiding too many particles that are too fine (below about 1 micron). For instance, sized weighting agents have been utilized with a particle size distribution such that at least 90% of the cumulative volume of the measured particle size diameter is approximately between 4 microns and 20 microns, with a weight average particle diameter (“d50”) of approximately between 1 micron to 6 microns. The sizing process, however, undesirably increases the material and energy costs involved with sized weighting agent. Another approach to reducing particle size while maintaining desirable rheology involves comminuting the weighting agent in the presence of a dispersant to produce particles coated with the dispersant. The weighting agent is comminuted to have a d50 below 2 microns to 10 microns. It is reported that the coating on the comminuted particles prevents the undesired viscosity increase that would be expected from use of particles with a reduced size. However, the coating and comminuting processes add undesired complexity and material and energy costs to utilization of the weighting agent.